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Across two decades and thousands of pages of reports, the world's most authoritative voice on climate science has consistently understated the rate and intensity of climate change and the danger those impacts represent, say a growing number of studies on the topic.

This conservative bias, say some scientists, could have significant political implications, as reports from the group – the U.N. Intergovernmental Panel on Climate Change – influence policy and planning decisions worldwide, from national governments down to local town councils.

As the latest round of United Nations climate talks in Doha wrap up this week, climate experts warn that the IPCC's failure to adequately project the threats that rising global carbon emissions represent has serious consequences: The IPCC’s overly conservative reading of the science, they say, means governments and the public could be blindsided by the rapid onset of the flooding, extreme storms, drought, and other impacts associated with catastrophic global warming.

"We're underestimating the fact that climate change is rearing its head," said Kevin Trenberth, head of the climate analysis section at the National Center for Atmospheric Research and a lead author of key sections of the 2001 and 2007 IPCC reports. "And we're underestimating the role of humans, and this means we're underestimating what it means for the future and what we should be planning for."

Underplaying the intensity
A comparison of past IPCC predictions against 22 years of weather data and the latest climate science find that the IPCC has consistently underplayed the intensity of global warming in each of its four major reports released since 1990.

The drastic decline of summer Arctic sea ice is one recent example: In the 2007 report, the IPCC concluded the Arctic would not lose its summer ice before 2070 at the earliest. But the ice pack has shrunk far faster than any scenario scientists felt policymakers should consider; now researchers say the region could see ice-free summers within 20 years.

Sea-level rise is another. In its 2001 report, the IPCC predicted an annual sea-level rise of less than 2 millimeters per year. But from 1993 through 2006, the oceans actually rose 3.3 millimeters per year, more than 50 percent above that projection.

Some climate researchers also worry that recent institutional changes could accentuate the organization's conservative bias in the fifth IPCC assessment, to be released in parts starting in September 2013.

The tendency to underplay climate impacts needs to be recognized, conclude the authors of a recent paper exploring this bias. Failure to do so, they wrote in their study published last month in the journal Global Environmental Change, "could prevent the full recognition, articulation and acknowledgement of dramatic natural phenomena that may in fact be occurring."

Conservative bias
The conservative bias stems from several sources, scientists say. Part can be attributed to science's aversion to drama and dramatic conclusions: So-called outlier events – results at far ends of the spectrum – are often pruned. Such controversial findings require years of painstaking, independent verification.

Yet some events in nature are dramatic, conclude University of California, San Diego, history and science professor Naomi Oreskes and Princeton University geosciences professor Michael Oppenheimer, co-authors of the study looking at the IPCC's bias. "If the drama arises primarily from social, political or economic impacts," they wrote, "then it is crucial that the associated risk be understood fully, and not discounted.”

IPCC Vice-Chair Jean-Pascal van Ypersele countered that "the mandate of IPCC is to assess where there is consensus, and to reflect the full diversity of views that are scientifically valid where there is not."* He conceded that by requiring teams of authors to agree upon a report’s text, the IPCC process is inherently conservative. Getting the balance right, he said in an e-mail, is "not always easy."

Oreskes, Oppenheimer and their co-authors argue the conservative bias pervades all of climate science.

But the underestimation by the IPCC is particularly worrisome, scientists say, because the organization is charged specifically with advising policy makers on the most relevant, accurate climate science.

Current science
Established in 1988 by the United Nations, the IPCC does no original climactic research. Its role is to review current science from around the world, then synthesize and summarize that data within comprehensive reports meant for policymakers.

Such assessments typically take five to seven years to complete in a slow, bureaucratic process: Thousands of scientists from around the globe, working as unpaid volunteers, first sift through the scientific literature, identifying trends and writing a draft report. That draft is reviewed and thoroughly revised by other scientists. Then a summary for policymakers, condensing the science even further, is written and subjected to a painstaking, line-by-line revision by representatives from more than 100 world governments – all of whom must approve the final summary document.

IPCC's four assessments – massive, multi-tome volumes released in 1990, 1995, 2001 and 2007 – are considered the gold standard in climate science. The fourth report earned both intense criticism from climate skeptics and the honor of the 2007 Nobel Peace Prize, shared with former Vice President Al Gore.

Pattern of under-projection
The pattern, said Oreskes in an interview, is under- rather than over-projection. "These data simply do not support the allegations by skeptics that scientists have been alarmists," she said.

One example: In November, scientists at the National Center for Atmospheric Research in Boulder, Colo., took a closer look at the computer models underpinning most climate predictions and concluded future warming is likely to be on the high side of climate projections.

Another example: This summer, NASA climatologist James Hansen co-authored an analysis of recent extreme weather across the globe. Hansen's team arrived at a strikingly different conclusion from an IPCC special assessment on the topic released just months earlier.

The Hansen study, published in August in the journal Proceedings of the National Academy of Sciences, concluded that rapid climate change over the past 30 years has loaded the dice in favor of extreme weather. The chance of extreme summer heat is now 13 percent higher than in 1980, the report found. Record heat waves seen by Europe in 2003, Russia in 2010, and Texas in 2011 would not have happened without human-caused global warming, it concluded.

Hansen's conclusion contrasted sharply with the hedging in the IPCC special assessment on extreme weather, published in March, 2012: "Confidence in projecting changes in the direction and magnitude of climate extremes depends on many factors," the report's summary for policymakers began. "Even the sign of projected changes in some climate extremes over this time frame is uncertain."

IPCC scientist and Pennsylvania State University professor of meteorology Michael Mann, who was not involved in the March IPCC report, said the IPCC missed an opportunity to provide politicians with a clear picture of the extent of the climate crisis. "Many scientists felt that report erred by underplaying the degree of confidence in the linkage between climate change and certain types of severe weather, including heat wave severity, heavy precipitation and drought, and hurricane intensity,” he said.

Communication burden
Not all blame IPCC for failing to produce sufficient alarm with policymakers, however.

Andrew Dessler, professor of atmospheric sciences at Texas A&M University, sees no need for the IPCC to do anything differently. "The burden of communication falls on policymakers, not scientists," he said. Scientists are responsible for providing the hard data. It is up to policymakers to lead, connect the dots, and explain to the public the necessity of responding to global warming.

But the consequences of a conservative bias by climate scientists can be significant, others like Oreskes note. A society blind to the full range of potential outcomes, particularly the most disruptive, can remain apathetic to the need for change, pushing hard decisions off into the future.

The melting Arctic ice pack may offer such an example.

Scientists suspect that a diminished Arctic ice pack has the power to shift weather patterns across the Northern Hemisphere. Less ice, the hypothesis goes, would weaken and shift the jet stream, causing it to block normal weather patterns and hold storms, dry spells and heat waves in place so that they pound a single location for days, weeks or months.

But with the ice supposed to stay intact until 2070 or later, this was largely a theoretical problem for the future.

No longer: Summer ice in the Arctic hit a new low in 2012, and now some scientists say there is likely a link between that meltdown and the record-breaking drought that caused an estimated $28 billion in damage across the United States, as well as the soggy summer that left Britain drenched.

Even Hurricane Sandy has a potential Arctic tie-in, with researchers suggesting that the anomalous strong high pressure weather system over Greenland, forcing Sandy ashore in October, was influenced by the ice cap's decline.

These events – and especially the rapidity with which they are occurring – were not foreseen by IPCC models.

Dismissed as outliers
Likewise, weather forecasters not associated with the IPCC, using short-term models, almost uniformly failed to predict the drought that gripped most of the United States this summer. The reason? The few computer models that did forecast a major drought were dismissed as outliers, according to a report by Climate Central, a science research and communication organization.

"It's like going to a doctor," said Princeton's Oppenheimer. "When data is weak you ask your doctor for his or her best judgment.

"That is what IPCC is supposed to do."

IPCC's Fifth Assessment will be released in four parts from September 2013 through September 2014. Reforms within the organization have resulted in a more demanding consensus process – one that may produce even greater caution in its conclusions, say several former senior IPCC authors.

"The next report shows every sign of being even more conservative than the previous ones," said Trenberth. Instead of 10 lead authors per chapter, 14 or 15 scientists will have a say, making consensus-building harder.

IPCC's internal rules and deadlines have also been tightened, preventing the inclusion of some of the most up-to-date studies, he added.

Input from contrarians
Penn State's Mann also feels that IPCC higher-ups, fearful of being attacked by climate skeptics, have "bent over backwards" to allow greater input from contrarians. "There's no problem in soliciting wide views that fairly represent … a peer group community," he said. "My worry is that they are stacking the deck, giving greater weight to contrarian views than is warranted by peer-reviewed literature."

There are indeed more authors for next year’s assessment – 831 as compared to about 500 for the 2007 report, said IPCC’s van Ypersele, “But there are many more chapters as well, because the scope of the fifth assessment is larger.” The resulting document, he said, will be “based on real science and not ideology.”

'Nature of research'
Underestimates will continue to characterize climate projections, cautioned Richard Somerville, IPCC scientist and Professor Emeritus and Research Professor at Scripps Institution, "But that's the nature of research," as it constantly discovers new possibilities.

Looking back at the 1950s when scientists first identified the climate problem, Somerville notes that the tone at the time "was not catastrophic at all, but rather curious to see how the climate system would react to a big spike in carbon dioxide emissions." Only over time did the full realization dawn on the scientific community that many of the consequences of climate change could be very serious and even catastrophic.

And that is what hasn't gotten across to the public, Somerville warned: a sense of urgency that, to most scientists, is now very clear.

"This is an urgency that has nothing to do with politics or ideology," said Somerville. "This urgency is dictated by the biogeochemistry and physics of the climate system. We have a very short time to de-carbonize the world economy and find substitutes for fossil fuels."

This article originally appeared at The Daily Climate, the climate change news source published by Environmental Health Sciences, a nonprofit media compan

* Correction (12/18/12): Earlier versions of this story overly compressed a quote from IPCC Vice-Chair Jean-Pascal van Ypersele. The full version of the quote is included in this article.